The two correct ways to say this at the present moment would be:
“I knew you were from Russia, but someone told me that you were not.”
“I knew you were from Russia, but someone told me that you are not.”
The first one is more “proper” than the second one, but they’re both correct. The reason is that the verb “knew” changes the following verbs to the past tense also.
The reason we have the choice of present or past in the second clause is that we have two choices of how to deal with it:
Purely through grammar rules (so “were” in past tense).
With the knowledge that the information is still true (so present tense “are”).
Then, Jamie, could the second sentence below be grammatically okay, right?
I knew you were from Russia.
I knew you are from Russia.
The fact that the listener is from Russia doesn’t change in the past when the speaker knew and now when the speaker is saying: it is always true - I think it matches #2 you gave:
Actually, in Korean, the second one is proper. The first one sounds like “I knew you were from Russia but now I know you are not.” Anyway, thank you for the good question & good answer. This part was always picky to me.